In none of these cases did the government officials have a warrant. Even among supporters of strict immigration-law enforcement, warrantless searches within the United States should be troubling as an infringement of our ancient freedoms. That they are occurring deep within the United States and without a reasonable connection to border crossing also means, taken in conjunction with more recent Fourth Amendment rulings, that the policy must be seen as increasingly unlikely to pass constitutional muster.

Under a law passed in 1946 (now codified as 8 U.S.C. §1357), the CBP claims the right to search people and places without a warrant if the purpose of the search is to find illegal aliens and the search occurs within a “reasonable distance” of an international border. In regulations passed in 1953, that “reasonable distance” was defined as 100 miles from any land border or sea coast. Almost without comment, Congress made large swathes of the United States into a region where the Fourth Amendment is seriously weakened.

As the ACLU has noted, roughly two-thirds of Americans live within 100 miles of the oceans or the borders. And while the law in question specifically exempts dwellings from warrantless searches, that is the only exception. If you find yourself anywhere else in the warrantless zone, including cars, boats, trains, backyards, and (as we have seen) buses, and you are liable to be stopped and searched if CBP suspects you may be an illegal immigrant. It could be even worse than that; the law also allows the CBP to extend its Fourth Amendment–violating search power to areas greater than 100 miles with the permission of its commissioner.

CBP’s website attempts to explain its power under §1357 as being fairly limited, but in doing so it elides certain points:

Border Patrol checkpoints do not give Border Patrol Agents carte blanche to automatically search persons and their vehicles, other then [sic] in the manner described above. In order to conduct a legal search under the Fourth Amendment, the agents must develop particularly probable cause to conduct a lawful search. Probable cause can be developed from agent observations, records checks, non-intrusive canine sniffs and other established means. Motorist’s [sic] may consent to a search, but are not required to do so.

It is hard to imagine the probable cause for stopping every car on I-93 in New Hampshire, or demanding the papers of every bus passenger in Bangor and Fort Lauderdale. One also suspects that most people have no idea that they can refuse to be searched at a roadblock or bus boarding, and CBP officers are under no obligation to educate them on this point. The result is effectively a system where the government may demand your compliance for next to no reason at all if you venture anywhere within the 100-mile zone.

In considering this policy, we should think about how we got here. It is entirely reasonable to say that the United States, like any country, has the right to decide who may cross its borders. It is an inherent part of sovereignty that no nation in the world would deny. And while the constitutional protections against unreasonable searches and seizure undoubtedly apply to Americans in America, no one could reasonably argue that they apply to non-Americans who have not yet entered America. The basic idea that we can search people who wish to cross our borders is a sound one.

But to stray from that basic, narrow concept drives us farther away from our constitutional protections and toward executive overreach. One part of the Constitution violated by this policy, the one the ACLU and others focus most on, is the Fourth Amendment. As the law in question here explicitly allows warrantless searches, the Fourth Amendment’s admonition against such things is clearly implicated: “The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.”

As the law in question here explicitly allows warrantless searches, the Fourth Amendment’s admonition against such things is clearly implicated,

As the text indicates, that amendment bans only unreasonable conduct by the government, which has led courts to a series of balancing tests to determine when a law-enforcement official has probable cause to execute a search or seizure without a warrant. The twists and turns of Fourth Amendment jurisprudence could fill a book, but one good summary for our purposes comes from the Supreme Court’s holding in Texas v. Brownin 1983. As Justice William Rehnquist explained in that case, “probable cause is a flexible, common-sense standard. It merely requires that the facts available to the officer would ‘warrant a man of reasonable caution in the belief,’ . . . that certain items may be contraband or stolen property or useful as evidence of a crime. . . . A ‘practical, nontechnical’ probability that incriminating evidence is involved is all that is required.”

The common-sense approach makes it hard to say that all of the CBP searches under §1357 are unconstitutional. If a CBP officer stops someone a thousand yards from the Mexican border who looks like he just swam across the Rio Grande, it is easy to say that there is probable cause for the search. Someone 90 miles from the border driving his own car is a less obvious case of law-breaking, absent some other incriminating information. A blanket inspection of people riding a bus in a non-border state such as Florida begins to look quite unreasonable indeed.

Americans also have rights that are not explicitly named in the Bill of Rights. The authors of the Bill of Rights recognized that rights are inherent in all people and do not originate with the government. Although the first eight amendments to the Constitution protect a particular set of these natural rights, the Ninth Amendment briefly reminds the reader that the set is not an exhaustive list: “The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.”

What, if anything, that means has been a subject of long-running debate. Proponents of government power claim that the Ninth Amendment is a truism, and that nothing in it restricts any power the federal government would ordinarily possess. This was the prevailing view of the Ninth Amendment through most of our history. The rebirth of originalism in the late 20th century, however, has led to a deeper understanding of the original public meaning of these words. From that point of view, the Ninth Amendment protects more of the people’s rights than one might think.

Professor Randy Barnett explains the unenumerated rights protected by the Ninth Amendment in a 2006 law-review article. In Barnett’s telling, the Ninth Amendment is “a meaningful check on federal power and a significant guarantee of individual liberty.” Even within the originalist school, there are several ways of interpreting the Ninth Amendment, all of which Barnett examines in turn, including his own preferred theory that “the Ninth Amendment was meant to preserve the ‘other’ individual, natural, preexisting rights that were ‘retained by the people’ when forming a government but were not included in ‘the enumeration of certain rights.’”

The right to travel has long been recognized among the natural rights retained by the people. As early as 1866, the Supreme Court held in Crandall v. Nevada that “all citizens of the United States . . . as members of the same community must have the right to pass and repass through every part of it without interruption, as freely as in our own states.” The Court made that right more explicit in 1958 in Kent v. Dulles, holding in an opinion by Justice William O. Douglas that “travel abroad, like travel within the country, may be necessary for a livelihood. It may be as close to the heart of the individual as the choice of what he eats, or wears, or reads. Freedom of movement is basic in our scheme of values.”

The Court examined the application of this right in the context of an international border in United States v. Martinez-Fuerte (1976). In that case, illegal aliens were arrested at a roadblock on I-5, 66 miles north of the Mexican border. The Court upheld the government’s actions. Admitting that the stops were seizures, Justice Lewis Powell wrote that the inconvenience to drivers was minor, and when weighed against the governmental interest, did not constitute a violation of the Fourth Amendment.

Members of the Court in those days were fonder of balancing tests than their 21st-century successors, who are more likely to recognize the importance of individual rights. Consider the case of Riley v. California (2014), in which the Court held that warrants are needed to search an arrestee’s cell phone, or Birchfield v. North Dakota (2016), which held that police may not force a suspected drunk driver to have a blood test without a warrant. Are these intrusions so much greater than being stopped on the road and forced to prove your citizenship?

Other factors complicate the legal and political landscape in ways that may not have occurred to the Court in 1976. The CBP website explains, accurately, that the law does not “give Border Patrol Agents carte blanche to automatically search persons” but requires “particularly probable cause to conduct a lawful search.” But what kind of particularized probable cause could justify questioning every single person on a bus or highway checkpoint?

Normally, probable cause involves some reasonable suspicion that the law-enforcement officer believes a crime has been committed. How is that manifested when the crime in question is illegal entry into the United States? For someone who arrived in the past hour or so, it is reasonable to assume that a CBP officer might discern evidence of his having just crossed a border illegally. For a checkpoint 50 or 100 miles in from the border, dealing with people who did not just get here but may have resided here for years, there can be no such evidence.

One suspects that the substitute for that evidence is the physical appearance of the person; specifically, whether he “looks Hispanic.” Recall the border stop in New Hampshire on I-93. The CBP was not looking for illegal immigrants from Canada, even though the proximity to the Canadian border was the legal justification for the stop. The people seized there were from Colombia, Brazil, Ecuador, and Mexico. What could have elicited particularized probable cause other than that these people looked like they were from Latin America? And what characteristic could have distinguished these illegal immigrants from legal immigrants, some of whom may even be American citizens?

The Court did not address this in Martinez-Fuerte in 1976, but it would surely do so now. A law that declares everyone a suspect but is exercised only against Hispanics has equal-protection implications that no court could afford to ignore. Combined with the Fourth and Ninth Amendment problems, the 100-mile warrantless-search zone is clearly of dubious constitutionality.

Even beyond the letter of the law, though, the exercise of such a power is at odds with some of the basic ideas of American liberty. We have no internal passports in this country, and the freedom to move around within its borders is one that most Americans take for granted. The phrase “papers, please” sounds to our ears like something from another place and time, as well it should. As long as the 100-mile zone for warrantless searches remains law, that freedom is diminished.